A 4-cylinder turbocharged engine may use short exhaust cam durations in order to reduce exhaust blowdowns of one cylinder from entering another cylinder during valve overlap at low to mid speeds. Such cylinder-to-cylinder interference is especially a problem on engines which use high valve overlap to achieve scavenging at low RPM high load (typically direct injection engines). Then, at high speeds, a longer exhaust cam duration may be used to increase peak power output. Such an approach is described in Grigo, Wurms, Budack, Helbig, Lange and Trost, Der neue 2,0-I-TFSI-Motor mit Audi valvelift system, Sonderausgabe von ATZ und MTZ, June 2008, pp. 30-34.
However, the inventors herein have recognized various issues with the above approach. In particular, the short cam durations may limit valve-timing-related fuel economy gains at part (partial) load. For example, the inventors herein have found that achieving optimum fuel economy benefits at part load may result in much longer cam durations, for example as much as 20 to 40 crankshaft degrees longer for intake and exhaust cam durations. However, as noted above, such longer durations may lead to still other problems of exhaust gas blowdown issues.
As such, in one approach, the above apparently conflicting issues may be addressed by using selective exhaust valve timing adjustments, particularly at part load conditions. For example, in one approach, a method for controlling an engine includes during a first operating range, operating the engine with a first exhaust valve duration at a first exhaust valve timing, during a second operating range, operating the engine with a second exhaust valve duration at a second timing, and during a third operating range, operating the engine with the first exhaust valve duration at a third timing, where engine load of the second operating range is higher than engine load of the first operating range, which in turn is higher than engine load of the third operating range, and where the first exhaust valve duration is longer than the second exhaust valve duration, and where the first timing is retarded to a greater extent than either of the second and third timings.
In this way, it is possible to utilize shorter exhaust valve durations at low engine speed, high load conditions to reduce exhaust blowdown interference and give increased scavenging and torque. Further, longer exhaust valve durations at part load are utilized to give improved valve-timing-related benefits and fuel efficiency. Moreover, operating exhaust valves with a longer opening duration and with retarded timings at partial engine load may increase fuel efficiency and emissions over a range of engine speeds.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.